1. Field of the Invention
The present invention relates to a conductive diamond electrode and an ozone generator using the conductive diamond electrode.
2. Background Art
Ozone is known as a substance, in the nature, having an extremely strong oxidizing power and has been widening its applications over various industries. For instance, ozone is utilized in waterworks and sewage plants for the sterilizing and decolorizing treatments. Another advantage of ozone is the nature that it turns to harmless oxygen through autolysis with time lapse. Ozone, therefore, is appreciated as a chemical used for sterilizing and decolorizing processes which are easier and safer in handling than the former processes by chemicals, without secondary contamination by chemical residues or reaction by-products, offering an easy post-treatment.
Well known methods of ozone production include the UV lamp process, the silent discharge process, and electrolysis process. The UV lamp process produces ozone through exciting oxygen by UV rays, available from a relatively simple unit, but the production volume is limited and therefore, popularly used for deodorizing rooms and cars. The silent discharge process is one of the most prevalent and commonly used ozone generation methods. It is widely applied for various purposes ranging from simple room deodorizing by a small-scale ozonizer to industrial water treatment by a large-scale ozonizer with an output capacity of several tens kilograms per hour. The silent discharge process uses oxygen gas or oxygen in air as feed material, and generates ozone through excitation by electric discharge.
The electrolysis process generates ozone in the generated anodic gas through electrolysis of water. Ozone gas can be obtained also through the electrolysis of aqueous solutions including that of sulfuric acid; however, if the electrolysis is carried out with ultrapure water as raw material applying a solid polymer electrolyte diaphragm, known as perfluorosulfonic acid cation exchange membrane, highly concentrated and purified ozone is obtained. The ozone production system by ultrapure water electrolysis has been widely used in the precision cleaning fields such as for semiconductor wafers or LCD substrates, since it applies ultrapure water as raw material and therefore, the generated gas contains impurities at an extremely low level.
Conventionally, lead dioxide (PbO2) deposited on the conductive porous metals including titanium by such means as electrolytic plating has been utilized as an anode for the electrolytic ozone process for its superior current efficiency in ozone gas generation. When ultrapure water is electrolyzed at a room temperature in a cell with perfluorosulfonic acid cation exchange membrane applied as the solid polymer electrolyte diaphragm and the lead dioxide applied as the anode, the current efficiency in ozone generation is usually 10-15%, even as high as 20% at a high current density. Although perfluorosulfonic acid cation exchange membrane is consumed with time during electrolysis, the amount of consumption is small, and therefore, a constant ozone output and operation safety can be maintained in more than two consecutive years of electrolysis run.
As above-mentioned, the lead dioxide anode shows a high current efficiency in ozone generation under a high current density and in continuous electrolysis operation, as well as good long-term stability, but the lead dioxide anode is susceptible to be reduced and deteriorative in a reducing environment. For instance, in a suspension of electrolysis operation, the lead dioxide on the electrode surface is easily reduced to lead hydroxide (Pb(OH)2), lead oxide (PbO) or lead ion (Pb2+) through chemical reactions with reducing materials including hydrogen remaining in the electrolytic cell or the electrolytic reducing reaction from cathodic polarization. Since these reduced substances have neither ozone generation ability nor electronic conductivity, such phenomenon is observed that the ozone generation capacity decreases in a resumed operation after suspension.
Therefore, the electrolytic ozone generator system applying lead dioxide electrodes usually has a mechanism to supply protective current in a range of 1/10- 1/1000 of electrolysis current normally applied to the electrolytic cell, to avoid performance deterioration during a cease of operation. Such mechanism, composed of a DC power source dedicated to the protective current supply, a battery, and a control system, constantly monitors state of the electrolytic cell so that no-current state does not occur even instantaneously. By this mechanism, lead dioxide anodes are protected from being exposed to a reducing environment even during the electrolysis operation cease; however, provision of such mechanism results in complicated working mechanism and configuration of the electrolytic ozone generator system, leading to a higher equipment costs.
Moreover, the lead dioxide anode contains a good amount of lead. Nowadays, use of lead tends to be reduced in all industrial commodities for its toxicity and legislative requirements, such as ROHS guide line (Refer to Non-Patent Document 1). The ROHS guide line is related to Directive 2002/95/EC of the European parliament and of the council of 27 Jan. 2003 on the restriction of the use of certain hazardous substances in electrical and electronic equipment.
On the other hand, it is known that the water electrolysis using conductive diamond as anode, where its conductivity has been induced by dopant such as boron added in the crystal structure, gives a current efficiency as high as 40% in terms of ozone output, which is fur higher than the electrolysis using the lead dioxide anode. Moreover, the conductive diamond anode is superior in chemical and electrochemical stability and gives no change in property and electrolytic performance even in the reducing environment where lead dioxide changes in quality and degrades. Accordingly, the protective current mechanism, which is essential in the electrolytic ozone generator system using lead dioxide anodes, is not required, leading to a simple equipment design. As a matter of fact, neither carbon nor boron, constituting the conductive diamond is the object materials in the ROHS guide line.
The conductive diamond electrode is filmed on a substrate by the processes as HFCVD and MWCVD. For either case, the substrate for the conductive diamond film is required to withstand a high temperature around 1000 degree Celsius in hydrogen atmosphere, which belongs to the coating environment, and to maintain the property of adhesion to the conductive diamond film, even under temperature changes during the treatment processes. As the substrate for the CVD synthetic diamond, silicon is commonly used, but more specifically, conductive silicon is typically used for the substrate of the conductive diamond electrode, which requires conductivity for functioning as a current distributor at the time of electrolysis, in addition to durability at the filming process. Such materials as carbon, titanium and niobium are also serviceable, but these are inferior in adhesion of diamond film or durability to a high temperature hydrogen atmosphere at the time of filming, showing defect that the conductive diamond film tends to peel off from the substrate during the electrolysis operation.
In the process of zero-gap water electrolysis, where ultrapure water with a very large electric resistance is directly electrolyzed, electrolysis is performed at the triple phase boundary of water/electrode catalyst/ion exchange membrane, as the electrochemical reaction field, where the electrode prepared by depositing electrode catalyst on a porous electrode substrate is pressingly attached to an ion exchange membrane. Also in the case of zero-gap water electrolysis using the conductive diamond electrode as electrode, water electrochemical reaction is performed at a triple phase boundary involving water, prepared by pressing the conductive diamond electrode against the perfluorosulfonic acid cation exchange membrane. In order to perform electrolysis for a long time stably, water must be constantly supplied to the contact part of the electrode catalyst with the ion exchange membrane, preparing a stable triple phase boundary.
If water is not supplied sufficiently to the contact part of the electrode catalyst and the ion exchange membrane and then the stable provision of the triple phase boundary is not achieved, not only raw material is not supplied to the electrochemical reaction field, but also the water content of the ion exchange membrane, as electrolyte, decreases. The water content of the ion exchange membrane has a close relationship with the conductivity of the ion exchange membrane; and decreased water content leads to a significant decrease in conductivity, further leading to increased cell voltage up to the state of electrolysis failure.
Moreover, cell voltage increase generates large Joule heat, resulting in overheating of electrode and electrolyte. Ozone has autolysis property, which is promoted by heat, and therefore, for better ozone yield, electrolysis should be performed at as a low temperature as possible.
In general, the formation of triple phase boundary in the zero-gap water electrolytic cell systems including the hydrogen generation system and the electrolytic ozone generator system is made in such a manner that an electro-catalytic metal or a metal oxide is deposited on the surface of a substrate having both conductivity and porosity, such as metal fiber or a porous carbon plate to prepare a porous electrode and then the porous electrode is pressed against the surface of a perfluorosulfonic acid cation exchange membrane; or the solution in which perfluorosulfonic acid cation exchange membrane monomer is dissolved is coated on the electrode surface and then the electrode is bonded with perfluorosulfonic acid cation exchange membrane by means of hot pressing, etc. to obtain sufficient conjugation. In general, the porous material applied is the material having porosity at a rate of 40% or more to perform smooth supply or liberation of water and gas.
As above-mentioned, silicon is the most suitable material for the substrate which can maintain a good adhesion with the conductive diamond film. For silicon, which is a brittle material, porositization in excess of 40% in porosity is difficult by fiberization or micro-pore formation, though it is possible for metals. Accordingly, in order to use silicon as the electrode structure performing zero-gap electrolysis, silicon substrate is processed to have convex-concave shapes and through holes to be functioned as a water supply path and a gas vent path, and the convex end of them is used as electro-catalytic portion of the triple phase boundary.
The shapes of convex and concave part are formed on the silicon plate material by lithographic patterning and etching by reactive gas or soluble solution. In this step, if sufficiently fine shaping is not achieved on the silicon plate, the area of the triple phase boundary, i.e., electrochemical reaction field will not be formed largely enough to provide adequate current supply to the electrolytic cell, leading to limited production amount of the targeted electrolysis product. Moreover, water content of the Non-Patent Document 1, called decreases due to stagnant electrolytic gases on the electrochemical reaction field; then the electric conductivity decreases, and the cell voltage will rise with time up to a difficult condition of current supply. In these view points, the surface of the convex part must be prepared finely so that water can be introduced to the entire surface and at the same time, generated electrolytic gas can be liberated from the electrode swiftly.
Preparation of the fine convex part on a silicon substrate, diamond film and the silicon substrate treated with diamond film on its surface is made by means of lithography. For the processing, expensive equipments with a high accuracy, such as a dry etching unit or a lithography unit are used, resulting in a high manufacturing cost. Moreover, the surface of the convex part prepared by the dry etching tends to be hydrophobic, and if used as the electrode for electrolysis, the surface is easily covered with electrolytically produced hydrogen or oxygen gas, interfering with the supply of water which is the material of electrochemical reaction.
When an electrode structure is fabricated by processing a substrate with extremely high planarity as seen in the silicon wafer commercially available, perfluorosulfonic acid cation exchange membrane tightly adheres to the electrode substrate filmed with conductive diamond. Then, water becomes hard to intrude into the entire surface of the convex part during the electrolysis operation, hindering electrochemical reaction. On the other hand, if the surface of the substrate is rough and not finely prepared, the area of triple phase boundary becomes large and advantageous in terms of water supply and liberation of electrolytically produced gases, since indented tops by the conductive diamond film having further fine indentation are formed on the surface of the convex part. However, since the perfluorosulfonic acid cation exchange membrane is forced to contact the electrode substrate by pressing, notches or pinholes may be developed on the perfluorosulfonic acid cation exchange membrane at the time of assembling the electrolytic cells. Therefore, the surface roughness of the electrode substrate must be controlled within a proper range.
Patent Document 1 describes that a conductive diamond electrode having a porous or mesh structure is effective to preserve supply and vent paths for water and gases; more in detail, it is suggested that in order to remove bubbles efficiently, pores should be provided to the substrate or the self-supported type conductive diamond electrode. In order to realize this, dry etching is required for the self-supported type conductive diamond electrode, which is weak in strength, by using a mask prepared by such methods as lithography; however, such processing method and processing equipment are expensive. Whereas, formation of pierced pores reduces the surface area capable of electrolyzing, and processing is complicated. For the self-supported type conductive diamond electrode, which is weak in mechanical strength, manufacturing is difficult.
In Patent Document 2, the self-supported type conductive diamond electrode in column state is disclosed as effective to increase the triple phase boundary area, but as with the case in Patent Document 1, the processing is complicated and difficult.